Replace hardcoded links to PyPI with extlinks

[hoefling]

This PR replaces referencing PyPI packages via hardcoded URLs to https://pypi.org/project/ by using sphinx.ext.extlinks plugin.

Old usage example:

`pytest <https://pypi.org/project/pytest>`__

New usage replacement:

:pypi:`pytest`

This PR partially addresses #9081.

[nicoddemus]

Hi @hoefling,

Thanks for the contribution, that looks neat.

However looking at the generated docs, I see things are not rendering correctly:

(The link points to the correct location though).

[hoefling]

@nicoddemus good catch! I've used sphinx master to test stuff out in a previous PR and forgot to revert it to the one in doc/en/requirements.txt. The changes in last commit amend that.

[hoefling]

Can be also verified by looking at the plugin list, the prefix is no longer there. It will be necessary later on though, when the doc build switches to Sphinx 5 :-)

[nicoddemus]

Thanks for the quick fix!

Can be also verified by looking at the plugin list, the prefix is no longer there. It will be necessary later on though, when the doc build switches to Sphinx 5 :-)

Not sure I follow, I still see it incorrectly rendered (also in other places), could you clarify?

[hoefling]

@nicoddemus also on hard reload/in private window? Might be a simple cache issue.

[nicoddemus]

Ahh sorry you are right, I didn't realize it was the same URL as before (RTD generates a unique URL per PR, and not per build as I assumed, which now that I actually think about it makes total sense).

Looks great, thanks a lot!